I'm going to take a shot at this: first, you were using a bi-pod on the rifle and I wonder if the barrel did not heat up enough with the 150s, which are hot and fast moving, or heat up just enough to throw the shots, the same way a warm/hot barrel throws its shots if the stock is impacting the barrel. Maybe the barrel cooled down enough when you went back to the 100 yd line so that she shot to poa.
Also, did you change the magnification on the scope when you went to the 200 yd line? I am not familiar with the Mueller line but believe them to be quality scopes, but yet you have a large range of magnification, and a big lens (50mm) that may cause your poi to move if adjusted.
I would not think a 150 gn 30 cal slug would move 6" in a 5-10mph cross wind unless a gust hit you as you fired, maybe, but maybe so.... You said your groups hit left 6" - how was the group, under 1" or did it spread out???
You shot your deer with a cold barrel, right? but your 200 yd target with a warm barrel?? Just thinkin'....
